Such valves are used for adjusting the steam flow rate admitted to the turbine. At small or intermediate lifts, the energy dissipated in such valves is very considerable. At small lifts where supersonic speeds are reached in the vicinity of the zone where the valve member comes into contact with its seat, shock waves and separations between the fluid and the walls of the valve arise and can cause the valve member to vibrate. At all lifts, the focusing of the flow sheet onto the axis of symmetry of the valve (see FIG. 1) gives rise to large pressure fluctuations that may be as great as 50% of the mean pressure, with these fluctuations propagating downstream where, under certain circumstances, they can give rise to damage in the pipework.
One solution for solving such problems consists in splitting up the flow sheet into a plurality of small jets. This can be achieved using a seat that includes a throttling collar with crenellations distributed around its periphery in the form of gaps separated from one another by teeth, as described, for example, in French utility certificate No. 76 12647, or in German laid-open application DOS-3 138 459.
Throughout the world, most large steam turbines are fitted with such valves, but it turns out that a certain level of vibration persists at partial load.